[oas2tson] Ref handling

[simoneb]

From @toomuchdesign:

In my own project I've finished implementing three resolution strategies for refs: import, inline, and keep. Currently, oas2tson implements inline.

• Import is interesting because it allows importing only the necessary subtrees.
• Keep doesn't resolve the refs (leaves them as they are), but it's potentially useful for registering schemas individually in Fastify and exposing an OpenAPI definition with the individual components defined (and not inline). There are some limitations in Fastify/Swagger that prevent achieving this currently, but I believe we'll get there in the future. Additionally, json-schema-to-ts is potentially able to resolve the types of schemas linked via ref.